Problem: On the first 7 quizzes of her chemistry class, Umaima got an average score of 78. What does she need on the next quiz to have an overall average of 80?
Let her score on the next quiz be $x$ The sum of all of her scores is then $7 \cdot 78 + x$ The same sum must also be equal to $8 \cdot 80$ Solve: $x = 8 \cdot 80 - 7 \cdot 78 = 94$.